LK555

Hi everyone,

I am currently taking part in a project where we aim to run a 1/10th scale electric motor from both batteries and Supercapacitors. The problem is, I am not that well versed in rc motors, and could use some advice on the size of motor to use.

This thing is going to be a fair bit heavier than a normal rc car due to the size of the capacitors, so is there anything I should know about the gearing and motor sizing I should know about?

We wont be going more than say 12 km/h, we are using one rear mounted motor and would assume brushless for longevity?

We also want to have a motor with a minimal power drain, as we want to maximise range.

Any suggestions would be greatly appreciated.

MarkA

iTrader: (21)

Yes, you'll probably want something brushless. How is the vehicle going to be controlled, i.e. with a standard RC radio or somehow else? You don't just plug a battery into a brushless motor and make it go - you need an ESC to run a brushless motor and that's where things will get tricky. A little more info and we might be able to be more helpful.

LK555

Hi again, sorry about late reply.

Right well I've narrowed down what have to work with:
we have 2 super capacitors which give a total voltage of 5.7 volts, and can supply a huge amperage
For the battery we have a 7.2 volt 3300 mAh battery
For Esc we have a viper eco27 (I think) which gives us a voltage range of 4.8 to 9.6 with a minimum motor turn limit of 27 turns.
We aren't wanting massive performance so we settled on a brushed motor and Esc due to cost etc.

The plan is to use a 1:1 ratio drivebelt to the rear diff, however this brings a problem. We are aiming to have the car run at pretty slow speeds compared to other rc cars, our max being 12 kph. However, with a 1:1 gear ratio this means our motor will be running at around 3000 rpm. This is pretty low from what I've been researching, as most motors are designed to be running way higher.

One suggestion was that we could use a motor with a higher number of turns, as at the same voltage it will be running at a lower rpm and more torque (which is good for us). Is this a correct assumption? Would say a 65 turn motor be more in our operating area?

Thanks for the feedback guys.

MarkA

iTrader: (21)

You are correct - rc car motors are not really intended to ever be ran at a 1:1 ratio so since what you are building is basically a form of a crawler, a higher turn crawler motor sounds like what you want. 65t probably won't hit a walking pace but the one at that link (or something like it) should suffice for your project.

LK555

I made a bit of an error when working out our motor rpm's for 1:1 gear ratio, so now it is a 3:1. The rear diff runs off a central drive wheel which uses a toothed belt to drive it. I counted 26 teeth on it, meaning we should use a 9 tooth gear on the motor, correct?

Basically we are re-using an older diff that came from another project, but still works fine, and we have been offered an 80 turn crawler motor, which according to the specs runs at 4450 rpm at max efficiency at 7.2 volts.

Our capacitors can only provide 5.7v, so using this motor our max rpm is 5.7/7.2 times this, which is 3522 rpm.

At our max expected speed, the motor is running at 2938 rpm. Still this is lower than our max efficiency, but I'm guessing with all of the inefficiencies of the drive train etc, this should be pretty reasonable?

Increasing the motor turns increases the rpm that its most efficient, however we cant really lower the gearing that much more due to the size of the gears, it seems that 9 teeth seems to be on the lower end?

Essentially, is the motor going to work, or would a lower turn motor be better?

MarkA

iTrader: (21)

You are assuming the ability of the motor to achieve anywhere near it's max rpm - those are always a fully unloaded rpm figure and you're looking at needing around an 80% loaded efficiency level to even hit 2900ish rpm. Add the mass of the vehicle, the rotating mass of the drivetrain, the friction of said drivetrain, etc., and on 5.7v, the thing might move on a 80t motor but would be outran by a toddler. As we'd say in racing, you need "more motor."

LK555

So would you guys recommend maybe a 55t then?

Also is a 3:1 reduction going to be ok for that?

MarkA

iTrader: (21)

I think that ratio is a good starting guesstimate and would say 55t will be the minimum amount of power you'll be able to get away with.

LK555

Thats great advice thanks.

Next problem, we have an aircraft style transmitter and receiver, which has 6 channels. We were hoping to use the undercarriage channel as a means of controlling a servo which will switch between the two power supplies, leading into the ESC.

Any suggestions about parts that would work for what we want? The connection between the battery and the ESC are tamiya connectors, so we plan to have a similar connector from the capacitors. These would all link in to a servo controlled switch between the two.

Are there any off the shelf parts that would save us making it up ourselves?

MarkA

iTrader: (21)

It shouldn't be too hard to wire a Y-connector between your two power sources into a switch but as to how to operate that switch, I'd say you're probably going to have to come up with that on your own.